This year, a smaller percentage of U.S. Internet users are contributing to social media sites -- uploading videos, writing blog entries, posting comments to news articles and writing online product reviews -- than in 2009, according to Forrester Research.

This means that companies need to find ways to re-engage those U.S. Internet users who have stopped participating on their social media sites.

Social media "creators," which Forrester defines as users who have a blog, upload videos and music and write articles, shrunk from 24 percent of the U.S. online population in 2009 to 23 percent in the second quarter of this year, according to data from the report.

"Critics," those who rate and review products, post comments on others' blogs, participate in discussion forums and collaborate on wikis, dropped from 37 percent to 33 percent.

Likewise, "collectors," Internet users who subscribe to syndicated feeds, tag Web pages and photos and in general organize content for the benefit of other users, fell from 21 percent to 19 percent, Forrester said.

Even "spectators," the folks who read, watch and listen to what the "creators," "critics" and "collectors" post online, dropped from 73 percent to 68 percent, according to Forrester.

To boost social media participation on their sites, companies should revise the online tools they offer for people to post comments, reviews, ratings and the like to make sure they are intuitive and easy to use, as well as offer participants rewards.

The good news is that social-networking sites continue to attract new people to social media, according to the report, for which almost 27,000 U.S. Internet users between the ages of 18 and 88 were polled online.

These social media rookies drove up the ranks of "joiners," who have social-networking profiles and visit social-networking sites, from 51 percent to 59 percent, according to Forrester.

A new category this year is "conversationalists," whom Forrester describes as people who post status updates on social-networking sites and microblogging services such as Twitter. Thirty-one percent of U.S. Internet users fall into this group.

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